Baltic Sea Environment Fact Sheet 2017, Published 30 January 2017
Author: Johannes Johansson
total runoff in 2015 was above the 1950 – 2014 mean value, as was the situation
in 2014. When looking at the period from 1950 to present, alternating between
dry and wet periods in the length of decades is common. One example of a dry
period occurred between 1968 and 1980 and the following decade (1981-1990) was
a rather wet one. The five year running mean has been increasing since 2004,
but it went down slightly with the 2013 runoff. It is still above the long term
mean value though.
total runoff to the Baltic Sea in 2015 was among the top 3 highest since 1950,
mostly due to a record high runoff in the northern parts of the Baltic Sea. A
large contribution to this overall high runoff is a record high runoff in the
Gulf of Bothnia with +48% compared to mean values. The Gulf of Finland also
experienced a high runoff of +11% while the runoff in the Baltic Proper and the
Gulf of Riga was below average, -5% and -6% respectively. The total runoff to
the Baltic Sea in 2015 was +12% compared to the long term mean of the annual
the period 1950 – 2015, the total runoff to the Baltic Sea shows no long-term
trend. This time period is characterised by dry and wet periods lasting for a
couple of years to a decade generally following the NAO index.
Figure 1. Total runoff deviation during
the period 1950 – 2015 to the Baltic sub basins based on annual mean values.
The mean runoff value and the 2015 value for each sub basin are written in the
top left corner in each panel. The black line represents the five year running
mean. The blue lines on the map represent all the major rivers within the
Baltic region. Click image to enlarge.
the assessment of this report, a new model, E-HYPE (http://hypeweb.smhi.se/), was introduced and used to estimate the runoff
to the Baltic Sea. E-HYPE produced runoff values for the period 1981-2015, for
the earlier period 1950-1980 the runoff is based on observations. Previous
years the latter period was simulated by a combination of observations and two
models HBV-model (Graham 1999) and Balt-HYPE (Arheimer et al. 2012). The HBV- and the Balt-HYPE model are no longer
operational, thereby the switch to the E-HYPE model.
to the change of data for this assessment, the results are somewhat different
from fact sheets in the past. In general, the E-HYPE model estimates a larger
runoff in relation to data published in the past, the overall patters are more
or less the same though.
is a quantitative background indicator of the freshwater discharge, carrying
the nutrients from the drainage areas to the coast.
is an important parameter for the change of pressure of nutrient supply due to
varying climate and climate change. Also change in land-use can influence
runoff. To evaluate the change of pressure of nutrient supply to the Baltic
region it is necessary to know the variability of runoff and normalise for this
natural variability. Dry periods, like the one during the 70’s, can mask the
marine eutrophication since the runoff was lower than average and hence also
the total load of nutrients. Extended dry periods should also lead to a slight
increase in surface layer salinity. During wet periods, the total nutrient load
(pressure) increases, making marine eutrophication (effects) even worse.
The indicator shows the annual runoff from drainage
areas integrated over the Baltic sub-regions. Runoff is governed by the
precipitation - evaporation on land areas and is also influenced by air
temperature. It is the sum of direct river and diffusive runoff. In all
sub-regions a strong seasonal, annual and decadal variability can be
distinguished. Especially wet and dry periods are characterising the runoff.
The 70’s was a fairly dry period compared with the 80’s and the later part of
the 90’s. Geographically, the runoff is of about the same size in the Gulf of
Finland and the Baltic Proper, whereas the Gulf of Riga contributes to a lesser
extent and the Gulf of Bothnia to a larger extent to the total runoff.
different sub basins are described by the deviation from their mean values
based on runoff during 1950 to 2014. The mean values and the 2015 values are
shown in each sub basin panel (Figure 1). Years with higher runoff compared to
the mean value are displayed as red bars and lower values with blue bars. A
five year running mean is displayed as a black line overlaying the bars in the
figure. The sub basins are displayed in the centre of Figure 1 and the sub
basins described are the Baltic Proper, the Gulf of Riga, the Gulf of Finland
and the Gulf of Bothnia. A figure with the sum of the Baltic Sea sub basins is
also included, partly to give an overview of the entire Baltic Sea and partly
to compare the annual changes to the NAO index.
the period 1950 – 2015, there is no obvious trend in the annual runoff, neither
in the total runoff to the Baltic Sea area, nor in the sub-regions. Instead,
this time period is characterised by dry and wet periods lasting for a couple
of years to a decade. During 2015 the runoff were above mean values in the Gulf
of Bothnia and the Gulf of Finland, while in the Gulf of Riga and the Baltic
Proper the runoff were below mean values, see Table 1.
Table 1. 2015 runoff values [m3s-1] are compared to the 1950-2014 mean of the annual averages for the sub basins in the Baltic Sea and the difference in % are displayed.
times, there have been similar features in the changes of runoff values for all
the sub basins. Other time periods, the changes are similar only in some of the
sub basins. All the sub basins had low runoff values in the early to mid 70’s
and higher in the end of the 90’s. In the Baltic Proper, the Gulf of Riga and
the Gulf of Finland, there were high values from the mid 50’s to the beginning
of the 60’s. In the Gulf of Bothnia, the Gulf of Riga and the Gulf of Finland,
there was an episode of increasing values during the 80’s while in the Baltic
Proper, there was a tendency of decreasing values. There were low values in the
Baltic Proper in the early 90’s while there were high values in the end of the
80’s and at the start of the 90’s in the Gulf of Riga, the Gulf of Finland and
the Gulf of Bothnia.
Figure 2. Panel A: The integrated deviations of the runoff to the
Baltic Sea. B: Total runoff deviation during 1950 – 2015 to the Baltic Sea. C:
NAO index during 1950 – 2015 based on winter mean values of the NAO index.
Positive index indicates stronger westerly winds bringing warmer and wetter
winters to Scandinavia. D: NAO index during the years 1864 – 2015. B-D: The
black line represents the five year running mean for each panel. Click image to enlarge.
Figure 2 displays the total runoff deviation during 1950 to 2015 to the Baltic Sea, both as integrated difference (A) (sum of abnormalities from the mean (1950-2015), starting and ending with 0 km3) and with bars displaying the year to year deviation from the mean (B). The integrated difference gives an idea of the total amount of runoff in the Baltic Sea. The NAO index during the years 1950 – 2015 based on winter mean values of the NAO index is presented in panel C. The black line shows the five year running mean. By comparing the running means of panel B and C between 1952 and onwards, the features correspond rather well with each other. Note though, that there are exceptions where the NAO index does not reflect the total runoff from the Baltic Sea e.g. in 1962, 1976 and 2010.
Figure 3. Correlation patterns between the NAO index and the total runoff to the Baltic Sea for different time periods. A: Time period 1950-2015. B: 1960-1979. C: 1990-2009. A low p-value (<0.05) is often used to determine a statistical significance of a dataset. A high r-value is used to determine how strong a correlation is, r-values >0.5 is considered to be a moderate correlation.
based on a positive correlation with a p-value of 0.0016 (Figure 3), the NAO
indices may be used to indicate general runoff to the Baltic Sea back in time.
This motivates the inclusion of the NAO indices for longer time series
presented in panel D (Figure 2). Furthermore, looking at certain time periods,
the correlation between the NAO index and total runoff deviation is rather good
(r=0.55, 1990-2009). For other time periods there seems to be no correlation at
all (r=0.12, 1960-1979), which indicates a more stochastic behaviour of the
cohesion between Baltic Sea runoff and NAO index.
discussions have focused on whether global warming would increase river runoff
in the Baltic Sea region, as suggested by most models and climate scenarios. A 500 year reconstruction of river runoff
has, on the other hand, indicated a decrease of the total runoff to the Baltic
Sea with increasing temperature as an effect of increased evaporation (Hansson
et al., 2010). There are clearly uncertainties
associated to river runoff that need to be further investigated.
Arheimer, B., Dahné J., and Donnelly, C. 2012. Climate change impact on
riverine nutrient load and land-based remedial measures of the Baltic Sea
Action Plan. Ambio 41, No 6, 600-612.
and B. Carlsson 1994. River runoff to the Baltic Sea 1950 – 1990. AMBIO Vol. 23,
No. 4-5, 280-287.
1999. Modelling runoff to the Baltic Sea. AMBIO Vol.
28, No. 4, 328-334.
Omstedt, A., Elken, J., Lehmann, A.,
Leppäaranta, M., Meier, H., Myrberg, K., & Rutgersson, A. 2014. Progress in physical oceanography of the Baltic Sea
during the 2003-2014 period. Progress
in Oceanography, 128, 139-171.
Hansson, D., Eriksson, C., Omstedt, A.,
Chen, D. 2010. Reconstruction of
river runoff to the Baltic Sea, AD 1500-1995. International Journal of
Climatology, 31.5 (2011): 696-703.
are collected at the BALTEX Hydrological Data Centre (http://www.smhi.se/sgn0102/bhdc/bhdc.htm) (1950-1980), whereas modelled data is obtained at SMHI using the E-HYPE
model (1981-2015) (http://hypeweb.smhi.se/). There might
be some inconsistencies regarding the result from the observations and the
model. The NAO indices are collected from https://climatedataguide.ucar.edu/sites/default/files/nao_station_djfm.txt
For reference purposes, please cite this Baltic Sea environment fact sheet as follows:
[Author's name(s)], [Year]. [Baltic Sea environment fact sheet title]. HELCOM Baltic Sea Environment Fact Sheets. Online. [Date Viewed], http://www.helcom.fi/baltic-sea-trends/environment-fact-sheets/.